Transducers are devices that convert electrical energy to mechanical energy, or vice versa. Transducers in audio loudspeakers, for example, convert electrical signals into mechanical vibrations that in turn create audible sound waves. Similarly, transducers are often used to generate high frequency ultrasonic waves for various applications such as medical imaging, non-destructive evaluation (NDE), non-invasive surgery, dentistry and the like.
Transducers generally create ultrasonic vibrations through the use of piezoelectric materials such as certain forms of crystals (e.g. quartz) or ceramic polymers. Piezoelectric materials vibrate in response to alternating voltages of certain frequencies applied across the material. U.S. Pat. No. 5,637,800 issued Jun. 10, 1997 to Finsterwald et al. and incorporated herein by reference, for example, discloses a transducer suitable for medical use that includes arrays of piezoelectric elements. Such a transducer is typically connected to electronics that drive the transducer via a coaxial cable or the like.
Piezoelectric elements are similar to common analog capacitors in that piezo elements generally include two electrodes separated by a piezoelectric material that functions as a dielectric. The overall capacitance of a transducer is dependent upon the area and the thickness of the piezo material. Because the piezo elements in many types of transducers (e.g. phased arrays, high density linear and curved arrays, high frequency linear arrays, multidimensional arrays and the like) are generally very small, such transducers generally exhibit relatively low capacitance. The low capacitance corresponds to a relatively high impedance compared to that of the drive electronics, which typically has an impedance on the order of 50–75 ohms. As is known in the art, the impedance mis-match between the transducer and the electronics results in inefficient transfer of electrical energy, undesirably high ringdown, excessive heat production (which can present a safety hazard if the transducer comes into contact with human skin), and the like. Hence, it is generally desired to match the impedance of the transducer to the impedance of the drive electronics. Impedance matching in this situation, however, can typically be quite difficult to accomplish in practice.
One method of decreasing the impedance of the transducer relative to the impedance of the electronics is to increase the capacitance of the transducer through the addition of external parallel capacitors or inductors. The addition of discrete elements, however, typically increases the cost, complexity and variability of the transducer. Other transducers have sought to reduce impedance by reducing the space between piezo elements (commonly called “kerf width”) such that the total quantity of substrate used is increased. Reducing kerf width, however, places piezo elements closer to each other, thus increasing undesirable cross-talk and resonance between piezo elements of the transducer. The overall performance of such transducers is therefore degraded.
Another strategy for reducing transducer impedance involves creating the logical equivalent of parallel capacitors. One such method is disclosed by Richard L. Goldberg and Stephen W. Smith, “Multilayer Piezoelectric Ceramics for Two-Dimensional Array Transducers”, IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, Vol. 41, No. 5, September 1994, pp. 761–771, incorporated herein by reference. This reference discloses a piezoelectric element formed by placing green (i.e. unfired) ceramic between two electrodes with “fingers” that create additional capacitance. Such an element has several disadvantages in practice, however, in that it is generally difficult to manufacture and that the differentiation shrinkage (i.e. thermal contraction) between green ceramic and the electrode materials during the firing process frequently results in cracks in the piezo element. Moreover, the conducting material in the electrodes frequently diffuses into the ceramic during the firing process, thus degrading the performance of the transducer. It is therefore desired to create an efficient and easily-manufacturable transducer having an impedance that can be matched to that of the drive electronics.